• SYSTEM DESCRIPTION
• EMISSION REDUCTION POTENTIAL
• TECHNICAL RELIABILITY
• COST CONSIDERATIONS
• EXPERIENCES AND MARKET PENETRATION
• CHALLENGES

Hybrid electric vehicles (HEV) have two power sources. One converts fuel into useable energy, and the other, an electric motor powered by an advanced energy storage device, lowers the demand placed on the first power source. When the two HEV power sources are arranged in parallel, one or both can be used, depending on the situation. The electric motor often can power the HEV alone in city driving or over flat terrain. When the hybrid is accelerating and climbing hills, the two power sources can work together for optimal performance. In a series configuration hybrid, a primary fuel is converted to electric energy by an internal generator set (usually engine and electric motor working as a generator).

A hybrid bus usually combines an internal combustion engine of a conventional vehicle with the battery and an electric motor (w1). (1).

Many configurations are possible for HEVs, but they always contain the following parts:

1. An energy storage system
2. A power unit
3. A vehicle propulsion system

The combination of the internal combustion engine and a battery and electric motor is the most common type and will therefore be focused on in this section. The internal combustion engine can be fueled by gasoline, diesel or other alternative fuels. In the longer term, there is some interest in fuel cell hybrid vehicles. For hybrids with larger secondary (rechargeable) energy storage, grid electricity charging (usually at night) is an option to increase range and reduce local emissions.

There are two basic HEV configurations (1). In both of them, the electricity comes exclusively from the engine:

• Series Hybrid Configuration: here the combustion engine drives a generator that feeds the electric motor or the battery.
• Parallel Hybrid Configuration: here both the engine and the electric motor are linked to the transmission so that either of them, or both at the same time, may provide the power to turn the wheels.

Hybrid systems have the following advantages (see also (w1)):

• Vehicles can run in local zero emission mode when required (e.g. in the city)
• Vehicles can also be powered "conventionally", e.g. outside the city and when higher speeds are required
• The combustion engine runs mostly under optimum conditions, thus reducing energy consumption and emissions
• The engine can be downsized compared to a conventional drivetrain with the same performance, meaning lower engine weight and higher efficiency
• Regenerative braking capability helps minimize energy loss and can recover the energy used to slow down or stop a vehicle
• Longer driving range compared to most battery electric buses.

Disadvantages on the other hand are:

• Equipment of two systems leads to increased weight and therefore additional energy consumption (although normally the energy savings from regenerative braking and other hybrid features more than offsets this penalty).
• Higher investment cost than conventional systems.

Two different systems may necessitate additional maintenance requirements.

"Hybrid buses can potentially reduce emissions by using less fuel and operating the engine at a narrow range of speeds and loads" (2). The engines can be smaller than conventional ones and in addition, regenerative braking allows the storing of energy in batteries that would otherwise be lost in braking (1).

"Diesel hybrid-electric vehicles offer reduced drive cycle emissions relative to conventional diesel buses, comparable to that achieved by conventional CNG buses and in most cases setting the in-use benchmark. Only emissions of nitrogen oxides from the hybrids failed to set the performance benchmark. ... The project confirmed significant fuel economy benefits of greater than 100 percent over a comparable CNG bus when operated on severe duty cycles such as New York Bus" (3).

Some emission comparisons with CNG buses can be found in (4).

Since hybrid vehicles have the potential to use less fuel, their use can also lead to lower greenhouse gas emissions compared to conventional diesel buses. According to (w1), "The first hybrids on the market will cut emissions of global-warming pollutants by a third to a half, and later models may cut emissions by even more".

It should be noted that these local ecological advantages do not necessarily justify a choice in favor of hybrid buses. As with CNG buses, a careful analysis is advisable, assessing to what degree local emissions can be reduced using improved diesel buses (advanced or retrofitted) or CNG-buses, which may be the quicker and more economic option.

As with electric buses, there is as yet very limited experience with the operation of hybrid-electric buses, and hence it is difficult to comment on their technical reliability.

However, according to (5), "hybrid-electric drive systems are being aggressively investigated as a means of facilitating several important transit bus design goals, including improved fuel economy, lower emissions, and lower maintenance requirements to reduce operating expenses." Furthermore, "if hybrid-electric propulsion allows for significant reductions in transmission and brake maintenance, fewer service bays and maintenance spares may be needed than with a similarly sized fleet of motor buses. On the basis of the performance of electric rail propulsion systems, mature, commercialized hybrid-electric drive systems should be quite reliable and durable. The braking capabilities of the hybrid-electric bus should result in dramatically lower wear rates and extended repair intervals of the mechanical service brakes as well."

On the other hand, the operation of two different systems, e.g. an electric one and a conventional one, may increase the maintenance requirements.

"In December 1999, NYCT contracted with Orion to purchase 125 hybrid buses at a cost of US$ 385,500 per bus. This figure well exceeds the price of a standard diesel bus, which costs approximately $270,000, and the latest price for CNG buses, which was $302,000 per bus for a 125-bus order in December 1999. NYCT does not anticipate the price of hybrid buses to approach the cost of diesel buses, but the agency says the price should come down to the level of CNG buses as the technology matures and the volume of orders increases. Critically, no depot modifications are necessary for hybrid diesel electric buses, which represents a dramatic cost savings over using CNG buses" (4).

According to (1), the higher cost of hybrid buses (about 50% above conventional buses), being at a very early stage of commercialization, is partly due to the following factors:

• The electronic control system
• The battery pack for energy storage
• The electric drive motor
• Recouping of R&D investments.

"It is possible hybrid buses will have lower operating costs than other bus technologies (excluding capital costs). The reasons include lower fuel consumption and longer brake pad life. However, these cost savings are offset by more expensive component costs (the power electronics), battery replacements needed over the life of the bus, and the need for more skilled maintenance personnel" (2).

This comparison, however, is based on light duty vehicles, not hybrid transit buses.

The US DOE claims that "the HEVs available for sale are very cost competitive with similar conventional vehicles. Any cost premium that may be associated with HEVs of the future can be off-set by overall fuel savings and possible incentives" (w1).

Toyota's "Prius", a four-door passenger car, is the first mass-produced gasoline-electric hybrid vehicle in the world. The Prius is currently for sale in Japan, the US and Europe where it boasts nearly double the fuel efficiency of conventional gasoline engines. It consists of a 1.5-liter gasoline engine and battery. In addition to conserving fuel, the Prius reduces CO2 by 50 percent and CO, HC and NOx by 90 percent below Japanese standards based on the Japanese test procedure. Testing by EPA indicates significant benefits under U.S. type driving conditions as well. Toyota is selling this vehicle at a premium of about 35 percent. They have stated that they could sell the vehicle at approximately the same price as a conventional vehicle and break even if production increases to about 200,000 units annually.

Eletra has developed a relatively simple diesel electric hybrid bus that it estimates to have 20-30% lower operating costs than a conventional diesel. Fuel consumption in actual operation was found to be from 33% to 39% better than a conventional diesel with a longer service life for the diesel since it operates at constant speed. Emissions of NOx are lower than Euro III and PM emissions are lower than Euro V.

The International Energy Agency (IEA) provides the following cost estimate table (6):

The figures above represent cost. Actual prices and availability may change according to commercial considerations as well as local vehicle service capability etc.

"Hybrid bus technology is just being developed, with a few early prototype systems in use in the US. Several transit properties in the US are experimenting with hybrid buses, but they are far from being a reliable commercial product." The DOE claims that it "is committed to making HEVs commonplace on American highways during the next decade and has created the HEV Propulsion Program to accelerate their development" (7).

A few hybrid buses have been demonstrated in national and EU funded local projects in Europe. Within the Clean Technologies Information Pool, the only project described in which hybrid buses are included is Diesel retrofits in New York City.

A hybrid bus demonstration project will soon get underway in Sao Paulo, Brazil. Hybrid buses will also be tested in Mexico City in 2004.

As noted for different markets and technologies, the availability for novel technologies on a certain market are subject to competent and active manufacturing and sales companies for the market in question. This may hamper introduction on markets where the producer do not see a reasonable, sustainable market for the product.

According to the DOE, the HEVs' "widespread penetration into the automotive market hinges mainly on the economics of producing a complex hybrid power system, rather than the inherent capabilities of the technology itself."

In the New York City experience, the following improvements were found to be necessary:

• Battery equalization and periodic battery "conditioning" are both required
• Programming must deliver a stable control system
• Some early component failures necessitated redesign
• Catalytic exhaust filter durability remains to be determined - important for emissions performance
• "Cleaner" small diesel engines are needed, with hybrid-specific engine programming

The DOE (3) finds key challenges to include the following:

• Adequate energy storage devices
• Advanced monitoring and control systems
• Reduced size, weight, and cost of power electronic devices
• Efficiency of hybrid power units (HPUs) must be increased while complying with emission standards
• More advanced mathematical models of propulsion system components are required.

Finally, it must be carefully analyzed whether the ecological advantages justify the additional expenditures in comparison to other options, such as CNG or improved diesel buses.